The present invention relates to a transparent plastic composition comprising polytetrafluoroethylene (PTFE) particles or a mixture of these PTFE particles and of mineral and/or organic particles, which composition can be used to form shaped articles, particularly sheets acting as a light guide and as a light scatterer. These sheets can be used in particular for light display systems, such as light display panels for advertising or information purposes, and for flat luminous screens useful, for example, for liquid-crystal devices.
Light display systems are known and generally comprise, mounted in an appropriate frame, an assembly comprising a sheet or panel of transparent plastic containing particles of a material able to scatter light.
The light source, generally fluorescent tubes, may be located near one face of the thermoplastic panel and light is observed, via the opposite face, which is transmitted directly and that which is scattered by the particles contained in the panel. In this case, the thermoplastic panel contains a large number of scattering particles in order to make it sufficiently opaque to mask the light source. The panel is incorporated into a box containing the fluorescent tubes. These tubes must be numerous enough to ensure good distribution of the light intensity. This box has the drawback of being thick and this device consumes a great deal of energy.
The light source may also be placed near one or more edges of the panel so that the latter is edge-lit. This type of panel then functions as a light guide. The light is therefore reflected and scattered by the scattering particles contained in the panel and some of the scattered light is observed through one or both faces of the panel. If the panel contains a large amount of scattering particles, the optical path followed by the light is short and part of the surface will be poorly lit or not lit at all. If the panel contains few scattering particles, the intensity of the scattered light is low. The scattered light intensity decreases with distance from the light source.
Attempts have therefore been made to find compositions made of a transparent thermoplastic that can be moulded into shaped articles, particularly in the form of sheets which can be used, for example, as panels in light display devices which, while containing a minimum quantity of scattering particles, produce a maximum and uniform scattered light intensity over the entire surface of the shaped article. It has been found, as described in European Patent Application EP-A-0,893,481, that it is possible to combine with the transparent thermoplastic, particularly polymethyl methacrylate, a defined amount (from 20 ppm to 1000 ppm) of a particular additive (a polyamide) in the form of particles having a mean size of between 0.4 and 200 xcexcm. Compared with the conventional solutions, this device reduces the energy consumption, provides an aesthetic advantage, by reducing the thickness of the light panel, and may lighten the supporting structure.
Attempts have also been made to improve the luminous efficiency of the shaped articles, particularly sheets.
The thermoplastic composition according to the invention for shaped light-scattering articles, based on a transparent thermoplastic formed from (meth)acrylic (co)polymer and on light-scattering particles, comprises polytetrafluoroethylene (PTFE) particles or a mixture of PTFE particles and particles of a mineral and/or organic compound, these light-scattering particles having a mean size of 0.5 xcexcm to 200 xcexcm and a refractive index which differs from that of the transparent thermoplastic by at least xc2x10.05 and being used in an amount, with respect to the total composition, of 5 ppm to 2000 ppm by weight.
The light-scattering particles preferably have a mean size of 2 xcexcm to 20 xcexcm.
The thermoplastic composition according to the invention preferably contains from 10 to 200 ppm and particularly from 30 to 100 ppm by weight of light-scattering particles.
The refractive index of the light-scattering particles preferably differs from that of the thermoplastic by at least xc2x10.1.
The composition according to the invention may also contain additives such as colorants.
The thermoplastic (meth)acrylic (co)polymer may, in particular, consist of an alkyl methacrylate homopolymer or of a copolymer derived from an alkyl methacrylate and at least one monomer containing one or more ethylenically unsaturated groups copolymerizable with the alkyl methacrylate.
As alkyl methacrylate, mention may especially be made of compounds in which the alkyl group has from 1 to 8 carbon atoms, for example methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate and butyl methacrylate. A particularly preferred monomer is methyl methacrylate.
The thermoplastic (meth)acrylic (co)polymer preferably comprises from 70 to 100% by weight of the main monomer: the alkyl methacrylate, and from 0 to 30% by weight of monomer(s) containing one or more ethylenically unsaturated groups copolymerizable with the alkyl methacrylate. This (these) monomer(s) containing one or more ethylenically unsaturated groups is (are) chosen, for example, from C1-C8 alkyl acrylates, styrene, substituted styrenes, acrylonitrile, methacrylonitrile, C1-C8 alkyl methacrylates differing from the main monomer, hydroxyalkyl acrylates and methacrylates, alkoxyalkyl or aryloxyalkyl acrylates and methacrylates, in which the alkyl group has from 1 to 4 carbon atoms, acrylamide, methacrylamide, acrylic acid, methacrylic acid, maleimides and alkylene glycol dimethacrylates, in which the alkylene group has from 1 to 4 carbon atoms.
The (meth)acrylic (co)polymers may be obtained by any known process, for example by suspension polymerization or bulk polymerization.
Preferably, the light-scattering particles consist only of PTFE.
The light-scattering particles may also consist of a mixture comprising PTFE particles, preferably as the major component, and particles of a mineral compound, such as titanium dioxide, barium sulphate and zinc oxide, and/or particles of an organic compound, such as crosslinked polystyrene or particles with a multilayer, for example bilayer, structure which are formed from at least one crosslinked polystyrene core.
The composition according to the invention may also be obtained by blending the thermoplastic, for example in granule form, the light-scattering particles (polytetrafluoroethylene and, optionally, particles of a mineral and/or organic compound) and, optionally, other additives such as colorants, these particles and additives generally being in the form of a masterbatch. This blend may be produced in any appropriate device.
The shaped light-scattering articles that may be manufactured from the thermoplastic composition described above may be obtained by various known moulding processes, particularly using extrusion, injection and compression, advantageously extrusion. Sheet products and moulded products are then obtained in various shapes.
The composition according to the invention is particularly suitable for manufacturing articles by extrusion. Now, in order for the shaped article to have scattering properties, the composition from which it is manufactured must contain particles which do not melt at the temperature used for the extrusion. The composition according to the invention, containing PTFE particles, is particularly suitable since these particles have a high melting point (320xc2x0 C.). This is also the case with the compositions which comprise a mixture of particles as described above, particularly that based on PTFE and crosslinked polystyrene.
The shaped articles may also be obtained directly in the form of sheets by the bulk polymerization of a mixture of (meth)acrylic monomers and optionally of their prepolymers, in the presence of light-scattering particles (polytetrafluoroethylene and, optionally, particles of a mineral and/or organic compound) and of the other optional additives, in a mould formed by two glass sheets (casting process).
For this bulk polymerization, it is possible to use any known free-radical initiator, for example diazo compounds, such as azobis(isobutyronitrile) (AIBN), and peroxides, such as benzoyl peroxide. The copolymerization generally takes place in the presence of a chain-transfer agent, such as diunsaturated monocyclic terpenes and monounsaturated bicyclic terpenes, such as terpinolene, and mercaptans, such as tert-dodecyl mercaptan.
It is also possible to add agents promoting mould release of the sheets, for example stearic acid and sodium dioctylsulphosuccinate, in the amount normally used.
The shaped articles according to the invention, particularly sheets, may also consist of a transparent thermoplastic, such as those mentioned above, having a concentration of light-scattering particles, such as those defined in the invention, which varies throughout the thickness of the article, the highest concentration being in a region close to the scattering surface. This concentration difference through the thickness of the article may be gradual, being in the form of a concentration gradient. This embodiment makes it possible to increase the light transmission-in that region of the article having a lower light-scattering-particle concentration and, consequently, greater uniformity of the scattered-light intensity over the entire area of the article, particularly in the regions remote from the light source. The light-scattering-particle concentration may also vary along the length of the shaped article, particularly a sheet, the lowest concentration being in the region close to the light source.
The sheets, obtained from the compositions according to the invention, may have thicknesses that differ depending on the envisaged use, and especially that range from 3 mm to 25 mm. In the case of light display systems for advertising purposes, the thickness is generally from 8 to 20 mm. In the case of flat light screens, it is generally from 3 to 6 mm.
Extrusion is a suitable process for manufacturing articles (sheets, for example) with the abovementioned thicknesses, and particularly with a small thickness. The extrusion process provides a thickness tolerance which is tight compared with other manufacturing processes, thereby ensuring reproducibility of the sheets, and therefore of the emitted luminous intensity, and making it easier to mount the sheets in frames with precise dimensions. This type of specification is particularly required for the production of flat light screens.
It is also possible to manufacture light-scattering panels which comprise, for example, a support made of a transparent thermoplastic, such as that mentioned above, and a scattering layer formed from the scattering thermoplastic composition described above, this layer being placed on one or both faces of the support. This product may be obtained by any suitable process, for example by coextrusion or coating. In this embodiment, the support may have a thickness of 2 to 25 mm and the scattering layer or layers a thickness of 20 to 1000 xcexcm.
The sheets obtained from the compositions according to the invention may be used in any light-display system, and especially that described in Patent Application EP-A-0,893,481.
The sheets according to the invention may also be used as flat light screens, for example for liquid-crystal (LCDxe2x80x94Liquid Crystal Liquid) screens.
Means can be used to improve the intensity of the light scattered by the shaped articles. In particular, in the case of sheets, these are, for example, screen-printed dots on at least one face of the sheets; or else these are films in the form of parallel adhesive strips which may or may not be uniformly spaced apart, the distance separating these strips possibly being shorter the further they are away from the light source. These films are placed on one face or on both faces (the face via which the scattered light is observed and/or the opposite face). Preferably, only the face opposite that via which the scattered light is observed carries a film in the form of parallel strips. These strips adhere to the surface of the panel by any suitable means. If the film on the opposite face to that via which the scattered light is observed is in the form of strips, a film or a sheet may be placed over this film in order to avoid light losses.
The following examples illustrate the present invention without however limiting its scope. The following abbreviations have been used:
The PMMA, in bead form, used for the manufacture of extruded sheets was that sold by Atoglas under the name xe2x80x9cOROGLAS(copyright) 9ELxe2x80x9d.
the PTFE particles used as light-scattering additive were those sold by DuPont de Nemours under the name xe2x80x9cZONYL(copyright) 1200xe2x80x9d having a mean diameter of 4 xcexcm and those sold under the name xe2x80x9cZONYL(copyright) 1000xe2x80x9d and having a mean particle diameter [lacuna] 11 xcexcm. The refractive index of PTFE is n=1.376 (xe2x80x9cPolymer Handbook, Wiley Interscience Publicationxe2x80x9d), thus being significantly different from that of PMMA (n=1.498).
the PS particles used as light-scattering additive were those sold by Sekisui under the tradename xe2x80x9cPS grade SBX-6xe2x80x9d. These particles have a mean diameter of 6 xcexcm and a refractive index n of 1.5916).
The reference (control) sheets with which the sheets of the invention obtained by casting or extrusion have been compared were cast sheets of PMMA having a thickness of 8 mm, 5 mm and 15 mm (the reference sheets are denoted respectively as R8, R5 and R15), sold by Atoglas under the name xe2x80x9cALTUGLAS ELIT(copyright)xe2x80x9d. These sheets contain, as light-scattering additives, a polyamide sold by Elf Atochem S. A. under the name xe2x80x9cORGASOL 2001(copyright)xe2x80x9d in an amount of 150 ppm.